In a commercial AC transmission and distribution system, a transformer is used. Just near a consumer's house, a pole transformer is used which transforms, for example, 6600 V (50 Hz or 60 Hz) to 200 V (see NON PATENT LITERATURE 1). Such a pole transformer has a thick coil as a conductive wire wound around an iron core, and therefore has a considerable weight. For example, a pole transformer with a diameter of 40 cm and a height of 80 cm has a weight of about 200 kg, including an insulating oil and a case.
On the other hand, for realizing a smart grid which is a next-generation power system, studies of an SST (Solid-State Transformer) are being conducted. For the SST, a high-frequency transformer is used (for example, see NON PATENT LITERATURE 2).
In recent years, there is a demand that, for example, output voltage (DC) of photovoltaic generation is stepped down to be used for a low-voltage power supply for measurement. There are cases where the output voltage of photovoltaic generation is as high as 1000 V. In order to step down such high voltage to about 100 to 200 V, an intervening device such as a step-down transformer is needed as in an AC circuit.